1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device and, more particularly, to a method for fabricating a semiconductor device having an interconnection layer using a low dielectric constant insulating film.
2. Description of the Related Art
As semiconductor devices shrink in dimension and increase in speed, the interconnection structure advances from a single-level structure to a multilevel structure. Semiconductor devices having a metal interconnection structure of five layers or more have been developed and produced. As miniaturization progresses, problems arise from a signal transfer delay caused by a so-called parasitic capacitance between interconnections and the interconnection resistance. Recently, the signal transfer delay caused by a multilevel interconnection structure lowers the speed of semiconductor devices much more, and various measures have been adopted.
In general, the signal transfer delay can be given by the product of the parasitic capacitance between interconnections and the interconnection resistance. In order to reduce the interconnection resistance, shift from a conventional aluminum interconnection to a low-resistance copper interconnection has been examined. The use of copper requires a buried interconnection structure because it is very difficult for the existing technique to process copper into an interconnection shape by dry etching similar to a conventional method. In order to reduce the capacitance between interconnections, an application of a low dielectric constant insulating film formed from an SiOF film by CVD, a so-called SOG (Spin on Glass having a SiCO composition) film by spin coating, or an organic resin (polymer) film has been examined instead of an insulating film by CVD using a conventional silicon oxide (SiO2).
The relative dielectric constant of the SiOF film can be generally reduced to about 3.4 (that of the conventional SiO2 film is 3.9). However, a further decrease in relative dielectric constant is practically very difficult in terms of the stability of the film. To the contrary, the relative dielectric constant of a low dielectric constant film such as the SOG film or organic resin film can be reduced to about 2.0, so that applications of these films are enthusiastically examined at present. In recent years, an SiCO film by CVD also becomes popular.
As a representative method for forming a buried interconnection, steps of forming an interconnection layer on an underlayer having a buried interconnection formed in advance are (1) formation of an etching stopper film, (2) formation of an interlayer dielectric film, (3) formation of a cap film, (4) formation of a mask for a via hole, (5) formation of a via hole, (6) removal of the mask, (7) formation of a mask for an interconnection trench, (8) formation of an interconnection trench, (9) removal of the mask, (10) formation of an opening in an etching stopper film, (11) formation of a barrier metal and seed Cu film, (12) formation of a plating Cu film, and (13) polishing and planarization by CMP and formation of a buried Cu interconnection. In general, the interlayer dielectric film is a low dielectric constant film, and the cap film is a conventional SiO2 film.
These days, multilevel interconnections of an LSI are stacked, and interconnection layers using low dielectric constant films are also stacked by repeating the above process scheme.
As the interlayer dielectric film using a low dielectric constant film, an SiCO film by spin coating or CVD is used as described above. The low dielectric constant film is known to be damaged in film deposition step (3), dry etching steps (5), (8), and (10), and mask removal steps (6) and (9) using plasma ashing and cleaning with a chemical, particularly in steps (3), (6), and (9). As for the SiCO film, an organic component (C component) in the film is influenced during these steps, and an Si—OH group as a water adsorption site is formed in the film to increase the hygroscopicity of the film. If water is adsorbed in the insulating film, it oxidizes a metal interconnection in subsequent steps, especially in heat treatment, and degrades the reliability of the interconnection.